1. Field of the Invention
The present invention relates to an organic electroluminescence element (organic EL device), an active matrix display including a liquid crystal device, and a method for producing the same.
2. Description of the Related Art
In recent years, flat panel displays including liquid crystal devices, organic EL devices or the like are widely used. FIG. 6 schematically shows the structure of an organic EL device 1. An anode 3, an organic EL layer 8 including a hole transporting layer 4, an emitting layer 5, and an electron transporting layer 6, a cathode 7, and the like are formed on a substrate 2 made of, for example, glass. Upon application of an electric field to the electrodes 3 and 7 through leads 9, the region of the emitting layer 5 sandwiched between the electrodes 3 and 7 is excited to emit light.
Commonly, when color display is desired, multiple pixels emitting different colors such as red (R), green (G), and blue (B) are arrayed on a substrate in orthogonal directions, for example, longitudinally and laterally, and the pixels are independently driven.
These pixels are driven by a passive matrix system or an active matrix system. Under the passive matrix system, anodes, an organic EL layer, and cathodes are sequentially formed on a substrate, and the intersections of the anodes and cathodes are driven to serve as pixels, while under the active matrix system, a TFT (thin film transistor) is formed for each pixel, and the pixel is driven by the TFT. Under the active matrix system, a TFT is formed for each pixel, which makes the structure more complex and increases the production cost in comparison with the passive matrix system. However, the active matrix system has advantages such as high definition, high quality image, and large screens.
FIGS. 7 and 8 schematically show the basic structure of a TFT. A gate 74, an insulating film 84, a semiconductor layer 76 made of amorphous silicon or polysilicon are sequentially stacked on a substrate 68, and a source electrode 80 and a drain electrode 82 are formed with a predetermined distance L between them on the silicon semiconductor layer 76. The drain electrode 82 is also connected to a pixel electrode 78.
Upon application of a predetermined voltage to the gate 74, an electric current flows between the source and drain through the silicon semiconductor layer 76, thus the electric current is supplied to the pixel electrode 78.
In an active matrix organic EL device, plural TFTs are combined to make a circuit. A basic example is that of two transistor type as shown in FIG. 9. A switching TFT 92 and a driving TFT 94 are provided for one pixel, and a capacitor 96 keeps the voltage between the gate and source of the driving TFT 94. These TFTs 92 and 94 are connected to a data line 100, a power supply lead 102, and a scanning line 104. When the switching TFT 92 selected by the scanning line 104 and the data line 100 is turned on, the capacitor 96 is charged to turn on the driving TFT 94, whereby a drain current flows to make an organic EL pixel 98 emit light.
Regarding the substrate, besides a glass substrate, a flexible substrate made of a resin film or a thin metal plate is proposed for displays (for example, Japanese Patent Application Laid-Open (JP-A) Nos. 7-78690, 2002-15859, and 2004-361774). However, particularly in cases where an active matrix display is made using a flexible substrate made of a resin film, the substrate significantly changes in dimension in the production process. When a TFT is made, holes (contact holes) are made thereby contacting the source electrode and drain electrode with the underlying semiconductor layer, so that the dimensional change of the flexible substrate is influential. For example, the position aberration of TFTs or the variation in the source-drain distance (channel length) significantly affects the pixel driving properties of the TFTs, which results in the deterioration in the display performance.
In order to prevent the position aberration of TFTs, there is proposed an active matrix substrate having transistors including functional lines each composed of a center line which has conductive surface and which is coated with an insulating layer and a semiconductor layer (JP-A No. 2003-174171). However, under the method, functional lines having special structures must be used, which increases the production cost or complicates the manufacturing process.